Knee joint prosthesis

ABSTRACT

A knee joint prosthesis which comprises a tibial component ( 4, 6 ) and a femoral component ( 2 ). The femoral component has condyles ( 14, 16 ) which act against the tibial component, directly or indirectly, during flexing of the knee. A cam ( 20 ) on the femoral component acts against a post ( 26 ) on the tibial component at high flex angles. The surface of the post which is contacted by the cam at high flex angles is convex when the post viewed generally perpendicular to the tibial bone contact and bearing surfaces, and the femoral bearing surface which is provided by the cam, where it contacts the convex surface of the post at high flex angles, is locally concave ( 32 ) when viewed along the surface of the post which contacts the cam so that the area of contact between the post and the cam is greater at high flex angles than at lower flex angles.

This invention relates to a knee joint prosthesis which comprises atibial component and a femoral component.

Flexing of a knee joint involves a combination of rotation andtranslation of the femoral component relative to the tibial component.The femoral component of a whole knee joint prosthesis has a bearingsurface provided by medial and lateral condyles. The surfaces of thecondyles are convex. The condyles bearing surfaces act against thetibial component, directly or indirectly through a meniscal component.It is common for a knee joint prosthesis to include a meniscalcomponent. This is frequently made from a material which permits lowfriction movement between the tibial and femoral components. Forexample, when the femoral component is formed from a metal such as acobalt chromium based alloy, the bearing component can be formed from apolymer such as ultrahigh molecular weight polyethylene. The face of themeniscal component which faces towards the femoral component willgenerally have a pair of recesses in which the convex bearing surfacesof the condyles can articulate as the joint is flexed. In some kneejoint prostheses, the tibial component will often have a planar bearingsurface which faces towards the femur. A meniscal component will thenhave a planar lower surface which is also planar so that the meniscalcomponent can slide relative to the tibial component as the joint isflexed. In other knee joint prostheses, the meniscal component can beallowed to rotate about the tibial axis but be fixed againsttranslation. In yet other knee joint prostheses, the meniscal componentcan be fixed relative to the tibial component.

The stability of the joint through moderate flex angles is provided bythe surface to surface contact between the curved surfaces of thecondyles and the recesses in the meniscal component. Ligaments extendingbetween the tibia and femur can control relative rotational andtranslational movement between the two bones. However, especially whenit is anticipated that the joint will experience large flex angles,additional stability can be provided by a post which extends from thetibial component in a direction away from the tibia, and a cam on thefemoral component which extends between the condyles at or towards theirposterior ends. The cam can contact the post, at least at moderate tohigh flex angles, and thereby restrict translation of the femoralcomponent relative to the tibial component. The post can be provided ona bearing component of the knee. The material of the bearing componentis often selected for its low coefficient of friction when in contactwith the metallic bearing surface of the femoral or tibial component.The properties of the selected material might not always be optimisedfor bearing the load which is applied by the cam at high flex angles.

While contact between a post on the tibial component and a cam on afemoral component is intended to restrict translation of the femoralcomponent relative to the tibial component, it will generally be desiredthat it should not restrict relative rotation between the femoral andtibial components. Accordingly, the area of contact between the cam andthe post will be kept to a minimum, especially as close as possible topoint contact. This can be achieved by arranging both the post and thecam to have rod or bar-like shapes, which are convex at the points wherethey are in contact when viewed along their lengths, and for them tointersect roughly perpendicular to one another.

The present invention provides a knee joint prosthesis in which thefemoral bearing surface is provided in part by a cam which extendsbetween the condyles, which is concave where it contacts a post on thetibial component at high flex angles.

Accordingly, in one aspect, the invention provides a knee jointprosthesis which comprises:

a. a tibial component which has a bone contact surface for contacting apatient's resected tibia, and an opposite bearing surface, and a postextending from the bearing surface in a direction generally away fromthe bone contact surface, and

b. a femoral component which has a bearing surface provided by medialand lateral condyles, and by a cam which is located between the condylesat or towards their posterior ends, in which the condyles of the femoralbearing surface act against the bearing surface of the tibial component,directly or indirectly, during flexing of the knee and the cam on thefemoral component acts against the post on the tibial component at highflex angles, and in which:

a. the surface of the post which is contacted by the cam at high flexangles is convex when the post viewed generally perpendicular to thetibial bone contact and bearing surfaces, and

b. the femoral bearing surface which is provided by the cam, where itcontacts the convex surface of the post at high flex angles, is locallyconcave when viewed along the surface of the post which contacts the camso that the area of contact between the post and the cam is greater athigh flex angles than at lower flex angles.

The knee joint prosthesis of the present invention has been found togive rise to the advantage that the ability of the post to bear theloads imposed on it by the cam at high flex angles is enhanced. This hasbeen found to arise from the larger area of contact between the cam andthe post at high flex angles than is available with prostheses in whichthe surface of the cam is not concave where it contacts the post. Thelarger area of contact contributes to greater stability of the jointprosthesis at high flex angles, for example above 120°, especially above130°, or above 150°. The provision of a localised concave region of thecam means that, at lower flex angles, the flexibility of the cam ispreserved by virtue of the fact that the contact angle between the camand the post is not increased.

Preferably, the ratio of the contact area between the post and the camat a flex angle of 150° to the said contact area when the flex angle is90° is at least about 2.0, preferably at least about 2.5. Preferably,the ratio of the contact area between the post and the cam at a flexangle of 145° to the said contact area when the flex angle is 90° is atleast about 1.3, preferably at least about 1.4.

The increased area of contact between the cam and the post can mean thatthe joint is more stable, and less prone to dislocation, than is thecase in which the contact between a cam and post is restricted to pointcontact.

The cam can be generally bar-like having a generally roundedcross-section when viewed along its length, at least around those partsof its periphery at which it contacts the post on the tibial component.The cam can be connected to the condyles at its opposite ends. It canalso be connected to other parts of the femoral component at otherpoints along its length, and possibly along its entire length.

The cam will appear to be concave when viewed sagittal (from one side)and transversely (along the femoral axis) in the region in which it isintended to contact the post at high flex angles, and convex around theother parts of the periphery at which it contacts the post at other flexangles. Preferably, the radius of curvature of the cam in the centre ofthe concave region, when viewed from one side, is at least about 25 mm,more preferably at least about 30 mm, especially at least about 37 mm.Preferably, the said radius of curvature is not more than about 60 mm,more preferably not more than about 50 mm, especially not more thanabout 43 mm. In a preferred embodiment, the radius of curvature of thecam in the centre of the concave region, when viewed from one side, isabout 40 mm.

Accordingly, it can be preferred for the said round cross-section of thecam to be interrupted in the region where the cam contacts the convexsurface of the post at high flex angles so that, in that region, thecross-section is flattened or concave. Accordingly, it can be preferredthat the cross-section of the cam is rounded at and towards its ends,and flattened or concave in a central region between its ends where itcontacts the convex surface of the post at high flex angles.

Preferably, the depth of the concave portion of the cam, measuredrelative to the surface of the cam at each side of the concave portion,is at least about 0.5 mm. Preferably, the said depth is not more than1.2 mm, more preferably not more than about 1.0 mm.

The transition between the convex surface of the cam and the concavesurface in the region in which it will contact the post at high flexangles should be carefully shaped so that the area of contact betweenthe cam and the post does not decrease significantly as the angle offlex increases to bring the concave region of the cam into contact withthe post. This can be achieved by making the edge of the concave regionrounded. The radius of curvature of the concave region rounded edge canvary around the concave region. The radius of curvature can be greatestat the posterior edge. For example, the radius of curvature at the atthe anterior edge of the concave region is preferably at least about 1.0mm, more preferably at least about 1.5 mm, especially at least about1.75 mm, for example about 2.0 mm. The radius of curvature at theanterior edge is preferably not more than about 3.0 mm, more preferablynot more than about 2.5 mm. The radius of curvature at the posterioredge of the concave region is preferably not more than about 6.0 mm,more preferably not more than about 5.0 mm, for example not more thanabout 4.75 mm. The radius of curvature at the posterior edge ispreferably at least about 3.0 mm, more preferably at least about 4.0 mm.

Preferably, the bearing surface on the cam is configured so that thesurface is concave where it contacts the post when the flex anglebetween the femur and the tibia is at least about 130°, more preferablyat least about 120°. The bearing surface on the cam will preferably thenbe concave where it contacts the post at all flex angles greater thanabout 120°, preferably at all angles greater than about 130°, forexample at all angles up to about 150°, preferably at all angles up toabout 155°. Preferably, the area of contact between the bearing surfacesof the cam and the post increases as the flex angle increases from anangle of not less than about 115°, preferably not less than about 120°,so that the area of contact reaches a maximum when the flex angle is notless than about 145°, preferably not less than about 150°. The area ofcontact will increase from a shape which approximates to a point, to ashape which is generally rounded, especially oval. As the area ofcontact increases through the increasing flex angle, the size of theoval area of contact increases.

The flex angle at which the concave portion of the cam bearing surfaceengages the post is determined by the orientation angle of the cam. Theorientation angle is the angle between the sagittal plane and a lineextending normally through the centre of the concave portion.Preferably, the orientation angle is at least about 15°, more preferablyat least about 20°. Preferably, the orientation angle is not more thanabout 30°, more preferably not more than about 25°. Preferably, theorientation angle is about 22°. When the flex angle is about 22°,contact between the post concave portion of the cam can occur at flexangles greater than about 120°, with contact area at a flex angle ofabout 155°.

When the cam is bar-like and extends between the condyles, it can bepreferred for the area of the cam at which it engages the post toincrease so that, when the area is at its maximum, it extends to a pointwhich is not more than 1.5 mm from the ends of the bar where it joinsthe condyles, preferably not more than 1.0 mm.

The tibial component can comprise an implant part and a bearing part.The implant part and the bearing part can be made from differentmaterials. For example, the implant part can be made from a metal suchas a cobalt chromium alloy or a titanium based alloy. The bearing partcan be made from a polymer such as an ultrahigh molecular weightpolyethylene (UHMWPE). The bearing part will often be fixed to theimplant part so that it does not move relative to it during articulationof the knee. For example, the bearing part can fit into a recess whichis defined by an upstanding wall, extending around at least part of theperiphery of the tibial part. Alternatively (or in addition), theimplant part and the bearing part can fit together by an arrangementwhich comprises at least one interdicting boss and recess. It willgenerally be preferred that the bearing part is fixed againsttranslation relative to the implant part, but can rotate relative to it.This can be achieved by means of a peg on the bearing part which isreceived in an axially expending bore in the implant part. For example,the implant part can include a hollow peg which can be received in anappropriately shaped cavity in the resected tibia. The space within thepeg on the implant part can receive a peg on the bearing part.Preferably the pegs on the two parts, and the recess within the peg onthe implant part, are conical with a circular cross-section.

A knee joint prosthesis in which the tibial component comprises implantand bearing parts, in which the bearing part is fixed againsttranslation relative to the implant part, is sold under the trade markSIGMA by DePuy Orthopaedics Inc of Warsaw, Ind., USA.

The post can be provided on the bearing part of an tibial componentwhich comprises separate implant and bearing parts. A bearing part whichincludes a post will generally be formed as a single piece whichconsists of the same material throughout (for example a polymericmaterial such as UHMWPE).

The height of the post will be selected to ensure that the cam will notride over the top of the post at high flex angles. The width and depthof the post (measured parallel to the sagittal plane) should besufficient to ensure that the post can withstand loads imposed on it bythe cam when the knee joint is flexed. The appropriate selection ofthese design features is known, for example as in the knee jointprostheses sold by Orthomet Inc under the trade mark AXIOM, and by Smith& Nephew Richards under the trade mark GENESIS (Posterior Stabilised).

Preferably, the surface of the post which is contacted by the cam athigh flex angles is convex when viewed from above (viewed generallyalong the tibial axis). Preferably, the surface of the post which iscontacted by the cam is flat, or possibly concave, when viewedsagittally (from one side).

It has been found that, by appropriate shaping of the cam and the post,the area of contact between the two can be at least about 50 mm²,preferably at least about 75 mm², for example from 80 mm² and up to 120mm² or more, at high flex angles (for example 140° or 145° or 150°).This is very much greater than can be achieved with knee prosthesisdesigns in which the surface of the cam is convex. As a result, the postis better able to bear the loads imposed on it by the cam at high flexangles. Furthermore, the increased area of contact between the cam andthe post can mean that the joint is more stable, and less prone todislocation, than is the case in which the contact between a cam andpost is restricted to point contact. Preferably, the area of contactbetween the cam and the post at lower flex angles can be not more thanabout 25 mm², more preferably not more than about 15 mm², for examplenot more than about 12 mm², or not more than about 8 mm².

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings, in which:

FIG. 1 is an exploded view of a knee prosthesis according to the presentinvention.

FIG. 2 is a sectional elevation through the femoral component shown inFIG. 1, on the line II-II.

FIG. 3 is an enlarged view showing the portion of the cam on the femoralcomponent in which the bearing surface is concave along the directionmarked by the arrow IV

FIG. 4 is a partial cross-section through the cam on the femoralcomponent on the line IV-IV.

FIGS. 5 a and 5 b are isometric and side elevation views of the bearingcomponent shown in FIG. 1.

FIGS. 6 a to 6 e are views from one side showing how the femoralcomponent of a knee joint prosthesis of the kind shown in FIG. 1articulates against the tibial component.

FIG. 7 is a graph which depicts the variation in contact area betweenthe femoral and tibial components of a knee joint prosthesis withchanges in the angle of articulation of the joint.

Referring to the drawings, FIG. 1 shows a knee joint prosthesis whichcomprises a femoral component 2 and a tibial component. The tibialcomponent comprises an implant part 4 and a bearing part 6. The femoralcomponent and the implant part of the tibial component are formed from acobalt chromium based alloy. The implant part of the tibial componenthas a downwardly extending peg 8 which can be received in anappropriately shaped cavity in the resected tibia. The upward surface 10is generally planar. It has a central opening 12 which communicates witha conical bore within the downwardly extending peg 8. A notch 9 isformed in the implant part of the tibial component to accommodateligament tissue.

The femoral component has medial and lateral condyles 14, 16, each ofwhich has a bearing surface with appropriately smooth finish. There is arecess 18 between the condyles. The recess is defined posteriorly by abar 20 which extends between the condyles. The bar is generally roundedin cross-section when viewed from one side along the medial-lateralaxis. It has a finished bearing surface around at least part of itscurved surface, at least in a central region which provides a cam.

The bearing part 6 of the tibial component is made from a polymericmaterial such as ultrahigh molecular weight polyethylene. Its lowersurface is planar and has a conical peg depending from it. The peg issized so that it fit snugly within the conical bore 12 in the implantpart of the tibial component. The peg and the bore have a circularcross-section so that the bearing part can rotate relative to theimplant part.

The upper surface of the bearing part has two concave recesses 22, 24formed in it in which the condyles 14, 16 can be received, and can slideduring articulation of the joint. A post 26 extends upwardly from thebearing part. The post has a bearing surface 28 on its posteriorlyfacing edge. When viewed from above along the tibial axis, theposteriorly facing surface of the post is convex. When viewed from theside, the posteriorly facing surface of the post is straight, orslightly concave.

One of the condyles 14 of the femoral component 2 is visible in FIG. 2.Also visible is the bar 20. The bar can be seen to have a generallyrounded shape when viewed from the side. The bar is formed integrallywith a web 19 which extends between the condyles, defining a recessbetween the condyles into which the post extends during flexing of thejoint.

The concave portion 32 of the bar is shown in more detail in FIGS. 3 and4. As shown in FIG. 3, the concave portion 32 has rounded transitions atits edge. The radius of curvature at the rounded transition at theposterior edge is greater than at the other edges. The radius at theposterior edge 34 is about 4.6 mm, and the radii at the anterior, medialand lateral edges is about 2.0 mm. The concave portion extends to withinabout 1.5 mm of the condyles at each end of the bar 20.

As shown in FIG. 4, the concave portion of the bar is rounded when it isviewed in a direction which is parallel to the bearing surface of thepost (when the concave portion and the post are aligned in contact withone another at maximum flexing of the joint). The radius of curvature isabout 40 mm. This is little more than the radius of curvature of thepost (when viewed from above along the tibial axis) with which it is incontact at high flex angles. When the concave portion of the bar isviewed from one side (as in FIG. 2), it is approximately straight.

The bar is polished on that part of its surface which is intended tocontact the post during articulation of the knee. There is a concaveportion 32 in that part of the bar which contacts the post at high flexangles.

FIGS. 5 a and 5 b show the bearing part 6 of the tibial component. Thebearing surface 28 of the post 26 is essentially planar towards itsupper end (as shown in FIG. 5 b). The bearing surface is alsoessentially planar along the medial-lateral axis, that is when viewedfrom above (along the line defined by the arrow “A”).

As shown in FIG. 6 a, at moderate flex angles (up to about 90°), contactbetween the femoral and tibial components is restricted to contactbetween the condylar bearing surfaces 14, 16 of the femoral componentwhich engage the recesses 22, 24 in the bearing part of the tibialcomponent. During flexing of the joint through moderate flex angles, thefemoral component can rotate and translate relative to the tibialcomponent. At a flex angle of about 90°, the cam provided by the bar 20engages the post 26 on the bearing component. The action of the postagainst the cam can control further translational movement of thefemoral component relative to the tibial component, in the planethereof, so that continued flexing of the knee is restricted largely toa pivoting motion, as shown in sequence in FIGS. 6 b to 6 d.

As shown in FIG. 6 b, at a flex angle of about 90°, the curved bearingsurface on the bar 20 engages the bearing surface on the post 26, thepoint marked “C”. At moderate flex angles (up to 115 or 125°), thebearing surface on the bar is convex where it contacts the post. Thearea of contact between the bar and the post is therefore restricted toa small area. This will be point contact (in the absence of anylocalised deformation of the components) when the each of the relevantbearing surfaces is convex at the point of contact. Continued flexing ofthe knee joint beyond 125° results in rotation and sliding of thefemoral component relative to the tibial component, about an axis whichis approximately fixed. As the flex angle increases, the shear forcesapplied to the post by the bar, parallel to the plane of the resectedtibia, increase.

Also as the flex angle increases and the bar 20 rotates relative to thepost 26, the concave portion 32 of the bar moves rotationally towardsthe post. The area of contact between the concave portion and the postincreases during continued rotation of the femoral component relative tothe tibial component until the area of contact reaches a maximum at aflex angle of around 150 to 155°. The variation in the area of contactwith angle of articulation is shown roughly in FIG. 7.

1. A knee joint prosthesis which comprises: a. a tibial component whichhas a bone contact surface for contacting a patient's resected tibia,and an opposite bearing surface, and a post extending from the bearingsurface in a direction generally away from the bone contact surface, andb. a femoral component which has a bearing surface provided by medialand lateral condyles, and by a cam which is located between the condylesat or towards their posterior ends, in which the condyles of the femoralbearing surface act against the bearing surface of the tibial component,directly or indirectly, during flexing of the knee and the cam on thefemoral component acts against the post on the tibial component at highflex angles, and in which: a. the surface of the post which is contactedby the cam at high flex angles is convex when the post viewed generallyperpendicular to the tibial bone contact and bearing surfaces, and b.the femoral bearing surface which is provided by the cam, where itcontacts the convex surface of the post at high flex angles, is locallyconcave when viewed along the surface of the post which contacts the camso that the area of contact between the post and the cam is greater athigh flex angles than at lower flex angles.
 2. A knee joint prosthesisas claimed in claim 1, in which the ratio of the contact area betweenthe post and the cam at a flex angle of 150° to the said contact areawhen the flex angle is 90° is at least about 2.0, preferably at leastabout 2.5.
 3. A knee joint prosthesis as claimed in claim 1, in whichthe cam is generally bar-like having a generally round cross-sectionwhen viewed along its length.
 4. A knee joint prosthesis as claimed inclaim 2, in which the said round cross-section of the cam is interruptedin that region where the cam contacts the convex surface of the post athigh flex angles so that, in that region, the cross-section is flattenedor concave.
 5. A knee joint prosthesis as claimed in claim 3, in whichthe cross-section of the cam is rounded at and towards its ends, andflattened or concave in a central region between its ends where itcontacts the convex surface of the post at high flex angles.
 6. A kneejoint prosthesis as claimed in claim 2, in which the cam is formedintegrally with a web which extends between the condyles, in contactwith the cam at a point where the cam does not contact the post duringarticulation of the joint.
 7. A knee joint prosthesis as claimed inclaim 2, in which when the maximum area of the cam which contacts thepost extends to a point which is not more than 1.5 mm from the ends ofthe cam where it joins the condyles.
 8. A knee joint prosthesis asclaimed in claim 1, in which the bearing surface on the cam isconfigured so that the surface its concavity is greater in the regionwhere it acts against the post when the flex angle between the femur andthe tibia is at least about 130° than in the region where it actsagainst the post at smaller flex angles.
 9. A knee joint prosthesis asclaimed in claim 1, in which the tibial component comprises a tibialimplant part for implantation in the tibia, and a bearing part, whichcan be positioned between the tibial implant part and the femoralcomponent.
 10. A knee joint prosthesis as claimed in claim 1, in whichthe depth of the concave portion of the cam, measured relative to thesurface of the cam at each side of the concave portion, is at leastabout 0.5 mm.
 11. A knee joint prosthesis as claimed in claim 1, inwhich the depth of the concave portion of the cam, measured relative tothe surface of the cam at each side of the concave portion, is not morethan 1.2 mm.
 12. A knee joint prosthesis as claimed in claim 1, in whichthe radius of curvature at the anterior edge of the concave region is atleast about 1.0 mm.
 13. A knee joint prosthesis as claimed in claim 1,in which the radius of curvature at the at the anterior edge of theconcave region is not more than about 3.0 mm.
 14. A knee jointprosthesis as claimed in claim 1, in which the radius of curvature atthe anterior edge is not more than about 6.0 mm.
 15. A knee jointprosthesis as claimed in claim 1, in which the radius of curvature atthe anterior edge is at least about 3.0 mm.